1. Field of the Invention
The present invention relates to an energization controller for controlling the ON/OFF condition of an H-type switching circuit for feeding chopped electricity to an electric coil of an electric motor.
2. Description of the Prior Art
A switching circuit for feeding electricity to a switched reluctance motor will be explained with reference to the drawings. The switched reluctance motor (hereinafter referred to as an SR motor) comprises a rotor configured so that pole sections protrude outwardly and a stator configured so that pole sections protrude inwardly. The rotor is an iron core comprised of laminated iron plates and the stator has a coil wound on each pole. The SR motor functions by operating each pole of the stator as an electromagnet and rotating the rotor by attracting each pole section of the rotor by the magnetic force of the stator. Accordingly, the rotor can be rotated in a desired direction by sequentially switching the power-ON state of the coil wound around each pole of the stator corresponding to the rotational position of each pole of the rotor. This type of SR motor is disclosed in Japanese Patent Laid Open No. Hei. 01-298940.
Because the power ON/OFF state of each pole of the stator is switched when each pole of the rotor is located at a specific rotational position in the SR motor, a magnitude of magnetic attraction force applied to the rotor changes abruptly by switching the ON/OFF state. Therefore, relatively large mechanical vibrations occur in the rotor and the stator and noise is also generated by this vibration.
In the above mentioned prior art technology disclosed in Japanese Patent: Laid Open No. Hei. 01-298940, a rotational position signal having a moderate leading edge and trailing edge is generated and is utilized to moderate the rise of current in energizing the electric coil and fall of the current in de-energizing the electric coil. This enables the suppression of vibrations and noise. However, since the rotational position signal is used, the effect of suppressing the noise is reduced when the rise of current in energizing the electric coil and the fall of current in de-energizing the electric coil become substantially fast when rotating the motor at a low speed. The current flow becomes very small and the torque generated becomes small since the power ON time for each energization is shortened when the rise of current in energizing the electric coil and the fall of current in de-energizing the electric coil become substantially slow when rotating the motor at high speed. Furthermore, efficiency may drop and the required torque may not be produced unless the timing for switching the ON/OFF control of the energization is changed corresponding to the number of revolutions and the required torque.
According to prior art technologies disclosed in Japanese Patent Laid Open Nos. Hei. 07-274569, Hei. 07-298669 and Hei. 08-172793, an H type switching circuit is used to control motor energizing current by a PWM to smooth the rise and fall of the energization and to control switching modes to improve torque.
For instance, as shown in FIG. 11a, the H type switching circuit comprises a first switching element 19a interposed between one end of an electric coil 1a and a first power line 18e, a second switching element 18b interposed between the other end of the electric coil 1a and a second power line 18f, a first diode D1 which is interposed between one end of the coil 1a and the second power line 18f and which permit current to be conducted from the latter to the former and a second diode D2 which is interposed between the other end of the coil 1a and the first power line 18e and which permit current to be conducted from the former to the latter.
A rotational driving current flows through the electric coil 1a as shown in FIG. 11a when the first and second switching elements 18a and 18b are both turned ON and a feedback current to the power source caused by an induced voltage of the electric coil 1a flows through the electric coil 1a as shown in FIG. 11b when they are both turned OFF. A pulsating current shown in FIG. 11c flows through the electric coil 1a by repeatedly turning the transistors ON and OFF as described above alternately by the PWM control. This switching mode will be referred to as "hard chopping" in the present specification. Energy generated by the electric coil 1a is supplied (regenerated) to the first power line 18e and the current is sharply reduced at time intervals during when the first and second switching elements 19a and 18b are both turned OFF as shown in FIG. 11b. Because pulsation of the current caused by the switching elements when they are switched ON/OF is large, pulsation of magnetic attraction force applied to the rotor of the electric motor is large, thus causing large vibration and noise.
A current whose pulsation is relatively small flows through the electric coil 1a as shown in FIG. 12c by alternately repeating operations of turning ON both first and second switching elements 18a and 18b as shown in FIG. 12a (similar to FIG. 11a) and of turning OFF only the first switching element 18a while keeping the second switching element 18b ON as shown in FIG. 12b. This switching mode will be referred to as "soft chopping" in the present specification. The current reduces moderately and hence the driving force of the motor and the attraction force in the radial direction are reduced moderately during the period in which the first switching element 18a is OFF and the second switching element 18b is ON as shown in FIG. 12b. Accordingly, noise and vibration are reduced when the motor is energized in the "soft chopping" mode.
The energization controller disclosed in the above mentioned Japanese Patent Laid Open Nos. Hei. 07-274569, 07-298669 and 08-172793 has realized the reduction of vibration and high torque by selecting the above mentioned "hard chopping" and "soft chopping" corresponding to the rotating condition of the SR motor.
However, because the energy generated by the electric coil 1a does not return to the first power line 18e during the time interval when the first switching element 18a is OFF and the second switching element 18b is ON as shown in FIG. 12b in the "soft chopping" mode, the rotor vibrates and jumps in the direction opposite from the driving direction due to a torque ripple and the like at the initial period of rotation right after starting to drive the electric motor. Thus, a reverse rotation occurs as shown in FIG. 13 and when this reverse rotation is large or is repeated in a short time, the current in the electric coil 1a may increase and the switching elements may be destroyed due to the induced voltage of the electric coil 1a caused by the reverse rotation of the rotor.
When an electric motor is mounted in a vehicle for driving the wheels, the rotor of the electric motor may well rotate temporarily in a reverse direction due to inverse torque applied to the wheels right after the drive of the electric motor is started to start the vehicle, in climbing a slope at low speed, in clearing a curb stone during driving the vehicle into/out of a garage or when forward/backward is quickly switched. Thus, it is desirable to reduce the possibility of destroying the switching elements.